1. Field of the Invention
The present invention relates to surge arresters in general and in particular to surge arresters of the metal oxide varistor (MOV) type designed for underoil mounting.
2. Background
It is well known that under normal service conditions the insulation of A.C. and D.C. electric equipment is subject to voltage within a fairly narrow range. However, for a variety of different reasons, such as lightning strike, one or another part of the electric system is liable to undergo a momentary increase in voltage much in excess of that used under normal service conditions, which consequently gives rise to overvoltage phenomena. Should the amplitude of this overvoltage reach a substantial value it may present a hazard to the insulation of the electric installations of stations, substations and power transmission lines. In particular, such overvoltage arising on the power transmission line are capable of damaging the insulation of the most expensive equipment items, i.e. electric machines, transformers, reactors, switching apparatus.
The suppression of the level of overvoltages arising on the power transmission line is accomplished by overvoltage protective devices, such as surge arresters, the application of which has now become virtually indispensable to the operation of high voltage power transmission lines. It should be noted that the operational reliability of electric equipment is largely determined by and dependent on the operational reliability of overvoltage protective devices.
Known in the prior art is an overvoltage protective device (see the U.S. Pat. No. 3,805,114 issued in 1974) comprising a column of non-linear resistors mounted within an insulating housing. Between the non-linear resistors and the inner surface of the insulating housing there is provided a gap. The non-linear resistors are made from zinc oxide-based material.
The non-linear resistors are characterized by a non-linear voltage-current relationship and act as a low resistance to the flow of high-magnitude overvoltage-induced currents, thus limiting the voltage across the terminals of the overvoltage protective device, and as a high resistance under normal service conditions, thus limiting the magnitude of current flowing through the device from the electric network.
Under normal service conditions a minimal current passes through the overvoltage protective device in a continuous manner. Upon the occurrence of overvoltage in the electric network as a result of the high non-linearity inherent in the resistors the passage of large-magnitude overvoltage-induced currents through the overvoltage protective device results in a pronounced increase in voltage at the point of connection of the overvoltage protective device to the electric network. The overvoltage across the equipment connected to the electric network in parallel with the overvoltage protective device is thus limited.
Upon cessation of the overvoltage impact on the electric network a sharp increase in resistance of the non-linear resistors takes place, due to which current through the overvoltage protective device is limited to the magnitude typical for normal service conditions.
Often an overvoltage protection device or arrester is installed adjacent transformer coils. In this configuration it operates in the same environment as a transformer coil, that is, underoil. During an overvoltage condition the arrester conducts current to ground protecting the adjoining transformer coils. In some conditions, the fault current is great enough to cause failure of the arrester. The excess current causes an arc to be generated internally in the arrester which may expel the disks and rupture the arrester housing. A catastrophic failure of an arrester in an oil filled environment is especially violent and may damage the transformer coils it was installed to protect.
In some prior art devices, spring clips were used to hold the disks under constant pressure. During excessive current conditions these springs could be expelled which in turn would short out transformer coils.
It is therefore an object to the present invention to provide an arrester wherein the means for maintaining the MOV disks under constant pressure do not present an electrical hazard under conditions in which they are expelled.